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I have a simple setup to try out Volumetric Lighting. I have a spot lamp and Volume Scatter set for the World. The cube has a window for the light of the spot lamp.

enter image description here

But no Volumetric Lighting. I have read most questions on this subject. But I have no idea where I am going wrong.

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Even though the text in the question says that there is a "Spot Lamp", the lamp in the linked file is set to "Sun".

Volumetrics will not work with a sun lamp. Sun lamp is not really a lamp in the sense that it's position on the scene won't affect the lighting, it is designed to give parallel rays from an infinite distance with no fallof... Just the opposite of what is needed for volumetrics.

Other light that won't work with volumetrics is Hemi

Change the lamp to any other type of directional lamp. Point, Spot or Area will work fine.

Also keep in mind that when you deal with volume scattering or absorption the light source will not be as intense, so be ready to bring up the intensity to compensate. Depending on the amount of scattering and/or volume absorption you might need very large values.

enter image description here

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    $\begingroup$ yes, you are right. It seems that GPU rendering does work on NVIDIA/Windows but not on MAC. $\endgroup$ – Old Man Dec 5 '16 at 17:05
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    $\begingroup$ AMDs are not the best cards for Blender... $\endgroup$ – cegaton Dec 5 '16 at 17:08
  • $\begingroup$ I had done some tests with a spot lamp but changed to sun lamp to try that. Apparently I uploaded the wrong blend file. Sorry $\endgroup$ – Old Man Dec 5 '16 at 23:06
  • $\begingroup$ Related: blender.stackexchange.com/questions/66041/… and: blender.stackexchange.com/questions/38830/… $\endgroup$ – cegaton Dec 6 '16 at 5:31
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    $\begingroup$ You can do volumetric lighting with sun, but you need to not use world volumetrics, but a mesh (e.g., a cube volume). $\endgroup$ – JakeD Dec 8 '16 at 15:37
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You do have volumetric lighting, but you don't have contrast to get the nice sunbeam effect. Try adding textures to give varying densities in the volume, then add something to cast shadows.

See this tutorial from Gleb Alexandrov for some great tips:

https://youtu.be/AXjE-t6dFZ8

Furthermore, try decreasing to size of the light source so that the shadows become sharper.

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